The present invention relates to an automatic tensioner for tensioning a belt such as a timing belt for an automotive engine and more particularly to a spring loaded screw type tensioner.
Japanese Patent Laid Open Nos. 56-83647 and 57-40149 disclose screw type tensioners applied to chain driven camshaft mechanisms. The tensioner has a shaft applied with torque by a torsion spring and an actuating rod having a thread engaged with a thread of the shaft. The shaft is urged by the spring in a rotational direction to project the actuating rod. When the tension of a chain decreases as a result of the expansion of the chain, the rod is projected by the rotation of the shaft to push a chain guide, thereby automatically tensioning the chain.
In the screw type tensioner, since a large force is continuously exerted on the thread, a square threaded screw device having a high rigidity is employed. In the square threaded screw device, the amount of particles produced by abrasion of threads increases, due to large friction between the threads. Therefore, it is preferable to use oil having a low viscosity rather than grease for a sufficient lubrication in order to wash down the particles. Accordingly, although the conventional tensioner can be applied to a chain drive device mounted inside an engine body in which engine oil splashes to lubricate the device, it cannot be applied to a timing belt drive device provided outside the engine body.